Pass Boundaries for Satellites

The Pass Break page of a satellite's Basic properties allows you to indicate the event that defines the boundary between passes (or revolutions) of an orbiting satellite. Most systems specify that a new pass, or orbital revolution, begins when a satellite passes through the inertial equator going north. A pass begins at the ascending node. The following diagram illustrates the ascending node of a satellite's orbit.

A pass break can be defined as the time when the satellite crosses a specified latitude boundary in either the ECF or ECI coordinate system. You can define the pass break by choosing the direction of motion (Ascending or Descending) when the satellite crosses the specified latitude. The default definition of the pass break is 0 degrees latitude in the inertial system on the ascending side of the orbit (the ascending node). The pass (or revolution) definition is a convention that satellite systems use to describe various periodic data.

If the specified latitude crossing is outside the satellite's range of motion, the north point or south point of the orbit is used as the pass break for specified positive or negative latitudes.

Due to the precessional and nutational motion of the Earth's spin axis, the crossing of the ascending node may occur at a time slightly different from the time when the satellite crosses the Earth-fixed equator.

Options for setting pass breaks

Option Description
Definition Options are:
  • Latitude - the Latitude crossing at which a new pass begins. This option is recommended for orbits that are not equatorial. A latitude of 0.0 deg for an inclined orbit coincides with the ascending or descending node.
  • Longitude - the Longitude crossing at which a new pass will begin. This option is not suitable for polar orbits.
Direction Specify either Descending (moving South) or Ascending (moving North) for the latitude crossing at the beginning of a pass.
Coordinate System Select the coordinate system in which the latitude or longitude should be measured from the drop-down menu; select from Fixed, Inertial, TrueOfDate, or TrueOfOrbitEpoch.
Partial Pass Measurement Indicate the method to be used to measure partial passes (typically used for reporting data):
  • By MeanArgOfLat - Passes are measured as the difference in the mean argument of latitude at the current time and the mean argument of latitude at the start of the pass break divided by 2π (argument of latitude = mean anomaly plus argument of perigee). Here, orbital elements are computed with reference to the object’s central body using its value for the gravitational parameter μ in the central body’s inertial frame.
  • ByTime - Time is used to measure partial passes.
  • ByAngle - Angles are used to measure partial passes.
For example, 0.5 corresponds to the halfway point of a single pass. If you choose to measure the pass by time, the time it takes the satellite to return to the start time is equal to the time it takes the satellite to complete the pass. If you choose to measure the pass by Angle, 0.5 corresponds to the point in the orbit 180 degrees from the start of the pass, which may not correspond to a time halfway between the start and stop times due to the eccentricity of the orbit.
Repeat Ground Track Numbering The path number in the repeat ground track cycle corresponding to the initial conditions and the number of revolutions in the repeat cycle. Path numbers are incremented concurrently with pass numbers, but are limited to a range of 1 to the number of revolutions in the repeat cycle. Once the path number reaches the number of revolutions in the repeat cycle, the next path number is 1. Path numbers are also maintained by the Maintain Pass # option.

Pass Numbering The pass number corresponding to the initial conditions of the satellite. If your new ephemeris start time is after the previous end time (or before the previous start time), STK estimates the pass number of the new start time based on the period of the orbit.
  • First Pass # - Enter an integer to identify the number at which pass numbering begins.
  • Maintain Pass # - STK continues the existing pass numbering sequence when a vehicle is repropagated or the epoch changes.

  • Use Propagator Pass Data - This setting should be used when the propagator is SGP4, which has its own definition for passes.
  • Date of First Pass - Manually override the first pass start time (normally the ephemeris start time). For more information on the options available for time instances see Time Options.

Determining the Orientation of Earth's Axis

The exact orientation of the Earth's spin axis is a function of three physical phenomena:

  • Precession is a slow rotation (period of 26,000 years) of the spin axis forming a cone about an average spin axis direction.
  • Nutation is a shorter period (18.6 years) wobble on top of the precessional motion.
  • Pole wander is the motion of the polar axis relative to fixed points on the Earth's surface, and is at least partially unpredictable.

Inertial coordinate systems are typically defined in such a way that the orientation of the spin axis and daily equinox at some point in time account for the effects of precession only (mean), or they account for the effects of both precession and nutation (true).